The present invention relates to a method and apparatus for adjusting the black level of the luminance component of a digitized picture signal in a television receiver or the like, to obtain a clearer picture.
In a television picture with few black areas, the black areas tend to stand out from their surroundings, creating a displeasingly stark effect. It is known art to control the black level so as to prevent this effect. The solid line in FIG. 1 shows the general form of the input-output characteristic of a black level control circuit, with input (Y-IN) on the horizontal axis and output (Y-OUT) on the vertical axis. The letters IRE indicate the amplitude scale standardized by the Institute of Radio Engineers. The input luminance signal is modified by compressing the darkest luminance levels and stretching the less dark luminance levels toward the black direction, thereby lessening the difference between the darkest areas and the surrounding less dark areas. The stretching also makes fuller use of the luminance scale by bringing darker luminance levels closer to the reference black level. In FIG. 1, for example, an input signal varying between thirty and seventy IRE units is stretched to an output signal varying from substantially zero to seventy IRE units.
The black level control scheme illustrated in FIG. 1 can be implemented by a circuit including discrete analog components such as transistors and resistors, as described in Japanese Unexamined Patent Publication No. H3-195274, for example. Due to the use of very-large-scale integrated circuits in recent television receivers, however, the modern trend is to digitize the picture signal so that it can be processed entirely with digital circuitry. Since the input-output characteristic shown in FIG. 1 is nonlinear and cannot by generated by simple arithmetic operations, it must be stored as data in a read-only memory (ROM). Since the input-output characteristic takes on various shapes depending on the values of control parameters, as indicated by the dashed lines in FIG. 1, there is much data to be stored. With straightforward storage schemes, the necessary memory capacity is very large, making the fabrication cost of the digital circuitry comparatively high.